Fix error when merging schema without a query

[balinthaller]

This fixes a case when you try to merge a schema which has no query defined with an arbitrary one.
Issues: #639 #659

TODO:

• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change. Include a description of your change, link to PR (always) and issue (if applicable). Add your CHANGELOG entry under vNEXT. Do not create a new version number for your change yourself.

[apollo-cla]

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[balinthaller]

Tests on older node versions seem to be failing because of graphql.js requiring a root Query for each schema to be merged. Does the GraphQL spec require this or is it a bug?

[freiksenet]

It's fixed in #527. Sorry that you had to do that extra work!

[geek]

I'm still running into this issue with v3.0.0...

{"errors":[{"message":"Query root type must be provided.","locations":[{"line":1,"column":12}],"path":["createPerson"]}],"data":null}

This is the result of merging 2 executable schemas, the first with the original structure:

     type Person {
        id: ID!
        firstname: String!
        lastname: String!
      }

      type Query {
        person(firstname: String!): Person!
      }

The second looks like this:

    const schema2 = `
      type Person {
        id: ID!
        firstname: String!
        lastname: String!
      }

      type Mutation {
        createPerson(firstname: String!, lastname: String!): Person!
      }
    `;

However, the problem goes away if I add a query to the second schema before the merge.

[stomvi]

Still get the error using graphql-tools v3.0.0.

First schema:

type Query {
  hello: String!
}

Second schema:

type Mutation {
  test: String!
}

Query:

mutation {
  test
}

returned data:

{
  "errors": [
    {
      "message": "Query root type must be provided.",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "test"
      ]
    }
  ],
  "data": null
}